The Mechanisms of Senescence 



in determining the senile decline. The powers of self-renewal 

 possessed by neurones apart from cell division have almost 

 certainly been under- rather than over-estimated. Neurone 

 regeneration in adult fish and amphibia can involve actual cell 

 replacement from a reserve of neuroblasts : 1 in adult birds and 

 mammals it is usually held to be limited to axon growth (see 

 Clemente and Windle 1955 for a review of the large literature). 

 Cell division, however, may not be the only means of nuclear 

 renewal — the appearance of 'binucleate' neurones in some old 

 animals has been taken as evidence of a process of reconstitu- 

 tion (Andrew 1955). Apart from this it is evident from observa- 

 tion that some neurones are capable of living and remaining in 

 function for 100-150 years, unless we postulate a system of 

 'reserve circuits' which has so far no evidence to support it. 

 The distinction drawn by Weismann between immortal germ 

 cells and mortal soma still persists in many of these assumptions, 

 in spite of the growing number of instances where differentiated 

 somatic cells in invertebrates are thought to give rise to germ 

 cells, or to structures having the potentialities of germ cells 

 (Brien, 1953). 



6-1 Senescence in Cells 



611 'IRREPLACEABLE' ENZYMES 



There is no self-evident reason why morphogenetic forces 

 acting upon cells, and inhibiting their free division, should lead 

 to their senescence. Mechanical and 'colloidal' interpretations 

 will not do — they fail to treat postmitotic cells as the dynamic 

 systems which they certainly are. A theory of 'mechanical age- 

 ing' in postmitotic cells could, however, be based upon the 

 exhaustion of specific cell constituents. It is reasonable to ask 

 how much of the senescence of such cells, if they necessarily 

 undergo senescence, is due to the existence of 'expendable' 

 enzymic or other intracellular structures which can be replaced 

 only at cell division. 



1 It has even been claimed that the Purkinje cells of the mammalian 

 cerebellum, the least likely of all such cells to do so, undergo a cycle of 

 periodic replacement from such a reserve (Baffoni, 1954). This is surely 

 either a fundamental discovery or an egregious error. 

 M 163 



